Found the answer to this problem ! You must have your phone on your person while going up stairs ! Also leave the app open in the back ground on your phone ! Stairs need two sensors operating one in your phone and one on your watch ! Try it out
Basically the only way I get it to count floor is to have my watch detect a workout. I only notice it is not counting when I am not actually paying attention to the floors that day. My watch also inaccurately counts floors during a 3-4 mile run with slight elevation changes. I've 24 floors on run before.